There is a continuing need for high altitude, long duration station keeping and reconnaissance aircraft. For example, the placement of a single cellular phone communications beacon at approximately 100,000 feet would provide complete coverage for a large area, the size of Southern California. There is no danger of interference with commercial aircraft flight and winds are low as compared with those occurring at lower altitudes. Such an aircraft would be cost effective if it could be kept on station for periods in the three year range, and if energy consumption were kept to a minimum. The use of a high altitude, long range and long duration aircraft with a reasonable price tag for weather reconnaissance and atmospheric research in general to replace the unpredictable untethered balloon is also much desired.
Consideration has been given to using electrically powered hovercraft for the station keeping mission, where the power is supplied by microwaves beamed up from ground stations. However, energy consumption is large and there is the danger of vehicles flying through the microwave beam. Furthermore, such vehicles could not be used in remote areas where electrical power is unavailable. Of course, any mission requiring significant changes in location would be out of the question.
A solar powered aircraft is another alternative. However, for station keeping the power requirements are so large that solar power alone is not practical. Additionally, the question arises as to how to power the vehicle during nighttime periods.
Dirigibles and blimps have been considered for such missions since they can provide significant fuel savings because no energy is required to support the vehicle structure and payload. However, buoyant vehicles using conventional propulsion systems could not meet the three year mission duration requirement.
An unconventional propulsion concept is disclosed by S. Andrews in U.S. Pat. No. 43,449 "Aerial Navigation" patented July 5, 1864. By alternately dropping ballast weights and venting off gas, the vehicle can be made to rise and fall. The center of gravity is varied causing a change in the angle of attack of the vehicle to provide trim as the vehicle rises and falls. The problem with this concept is that there is only a limited amount of gas and number of ballast weights that can be released, thus, severely limiting the range of the craft.
A more advanced version of obtaining forward thrust by alternately ballasting and deballasting, causing the vehicle to porpoise through the water, is disclosed in U.S. Pat. No. 3,157,145 "Underwater Glider" by F. E. Farris et al. Although designed as a submarine, the concept is applicable to aircraft. Here, the ballast tanks are alternately filled with water and then pumped out with air causing the vehicle to alternately descend and ascend. Hydrodynamic surfaces are then used to provide lift forces for trimming. While this propulsion system could be applied to buoyant aircraft by using air and helium, or hydrogen gas combination instead of water and air, there is a finite number of cycles limiting the duration of any mission.
Thus, it is a primary object of the invention to provide a propulsion system for a buoyant aircraft that can maintain the aircraft aloft at high altitudes for long periods of time.
It is another primary object of the subject invention to provide a propulsion system for a buoyant aircraft that provides forward thrust by changing the buoyancy of the aircraft.
A further object of the subject invention is to provide a propulsion system for a buoyancy aircraft which maintains a constant mass of buoyant gas.